1. Field of the Invention
The present invention relates to a lens system, and particularly relates to a low-cost, small-size and yet high-performance imaging lens system suitable for use in a portable device.
2. Description of Prior Art
As electronic techniques develop constantly, portable electrical devices, such as mobile phones, notebooks and personal digital assistants (PDAs), are commonly integrated with optical techniques. A typical sample is that a mobile phone is combined with a zoom lens to realize imaging function. However, the portable imaging devices generally require its lens system to be not only maintaining a high level of optical performance but also low cost and compact, due to the limited space of the portable imaging devices.
As an image pick-up device, the conventional spherical glass lenses have been widely adopted in optical technology because there is a wide range of materials available for making such lenses and the color aberration thereof can be more easily corrected. However, the spherical glass lenses have the disadvantages of uneasy correction of spherical aberration and astigmatic aberration when they are used in applications requiring a small F number and a large wide angle of field. On the other hand, the spherical glass lens will be disbennifit in reducing the dimensions and the cost of the image pick-up device. To improve the above-mentioned drawbacks existing in the conventional spherical glass lenses, aspheric plastic lenses or aspheric molded glass lenses have been used to obtain better image quality and smaller dimension and lower cost.
The material of the lens generally includes glass and plastic. A glass lens has better transparent capability than that of a plastic lens, but the cost and weight of the glass lens is inferior to the plastic lens. Thus, currently, most imaging lens systems for compact imaging devices, such as camera phones, generally consist of a combination of both plastic lens and glass lens to provide high level optical performance and low cost and weight. In the current marker, most imaging lens systems are composed of one glass lens element and two plastic lens elements, or one glass lens element and three plastic lens elements for compact lens systems.
U.S. Pat. No. 6,441,971, entitled “Compact Lens with External Aperture Stop”, discloses an imaging lens system composed of one glass lens element and two plastic lens elements. Specifically, the imaging lens system of the '971 patent includes, from the object side thereof, an aperture stop, a distal meniscus lens element made of glass, and first and second aspheric lens elements made of plastic. The meniscus lens element has a convex surface facing the object side, and is adapted to reduce lens temperature sensitivity and provide most of the focusing power while the first and second aspheric lens elements mainly provide aberration compensation. However, the lens system disclosed in the '971 patent is only suitable for an imaging pick-up device with a resolution under 640*480 pixels, it does not satisfy the requirement of higher resolution. Although the conventional lens system composed of one glass lens element and three plastic lens elements can satisfy the requirement of high resolution, the cost and the weight still are very high. Accordingly, in order to satisfy low cost and portability but having a high resolution desired by the market, further improvements on such an imaging lens system are needed.